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#55020 - 12/20/15 11:55 PM Do orbiting bodies accelerate?
Bill S. Offline
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Registered: 08/20/10
Posts: 3570
Loc: Essex, UK
This is just a series of thoughts leading to a “conclusion”. I would appreciate comments/criticism, please.

1. Velocity is a vector which includes speed and direction.
2. Acceleration is change in velocity.
3. Change of speed with constant direction = acceleration.
4. Change of direction with constant speed = acceleration.
5. A body orbiting at constant speed is constantly accelerating.
6. Gravity is not a force that holds an orbiting body as though it were on a string.
7. Gravity alters the geometry of spacetime such that it becomes curved.
8. The curve thus formed is a geodesic, and is defined as the most direct path from A to B in curved spacetime.
9. Thus, a geodesic is equivalent to a straight line in flat (non-curved) spacetime.
10. A body travelling at constant speed in a straight line is not accelerating.
11. It should be reasonable to argue that a body following a geodesic at constant speed is not accelerating.
12. It should, therefore, be reasonable to conclude that an orbiting body is not accelerating.
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#55026 - 12/21/15 03:56 AM Re: Do orbiting bodies accelerate? [Re: Bill S.]
Orac Offline
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Registered: 05/20/11
Posts: 2819
Loc: Currently Illinois, USA
You are totally correct but still totally wrong and you can thank your teachers and classical physics smile

You sort of reach the right conclusion in (12) after a couple of wrong turns. The problem is acceleration is defined, as are all things in classical physics static universally and global.

You then switch to GR curved spacetime without changing your definition of acceleration. The classical physics version of acceleration is totally ambiguous and meaningless in the GR framework except on a case by case conversion.

When you look at Bill S standing on the earth, you will conclude the same that gravity is accelerating you towards the ground using classical physics smile

Definition problem again .. redefine acceleration properly to fix it

https://en.wikipedia.org/wiki/Proper_acceleration
Quote:
In relativity theory, proper acceleration[1] is the physical acceleration (i.e., measurable acceleration as by an accelerometer) experienced by an object.

If you can't measure an acceleration you aren't smile

Don't mix frameworks as you can't, the definitions of spacetime differ and you will just get confused. Your statement (12) is correct in GR because you can't measure any acceleration with an accelerometer not because of any classical physics calculations. It is true by definition in GR and requires NO calculations.

GR is not an addition to classical physical physics it redefines everything. I am going to suggest you put the word "proper" infront of terms when you wish to use GR/SR so you remember it's different to it's classical physics counter part, so proper time, proper distance, proper acceleration etc.

So in classical physics you standing there you have an acceleration of 9.8m/s towards earth. In GR/SR you have a proper acceleration of 0m/s and it doesn't really require any explaining although countless webpages seem to want to try and just confuse people. It's that way because the frameworks define it that way ... the end !!!

People get fuel consumption of 1 gallon per mile does not equal 1 litre per kilometre yet we can't seem to get them to understand the above. Just because GR/SR gives different answers to classical physics nothing is wrong the unit definitions aren't the same.

All of the above is also the answer to the other problem with speed of light you were playing with and I was trying to get you to self realize. The speed of light in relativity is BY DEFINITION the same to all observers and here you were talking about different speeds of it without considering that was just from your assumption or inferring from your reference point. You were doing a Dave Profitt and going with your observation as a global truth ... try putting yourself as an observer where you see it slowed. Remember measuring or seeing something only ever makes it true from that reference frame.

What I might start doing is when you mix classical physics and relativity just simply prompt you with "Does 1m/g = 1km/l" as a reminder.

So your two problems are true by definition and can only be equated on a case by case basis with different adjustments under classical physics. Why ... because spacetime is static in one framework and relative with global zero frame forbidden in the other. There is no simple identical conversion between the two frameworks, in all situations that will hold.


Edited by Orac (12/21/15 05:08 AM)
_________________________
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#55053 - 12/24/15 07:10 PM Re: Do orbiting bodies accelerate? [Re: Bill S.]
paul Offline
Megastar

Registered: 03/21/06
Posts: 4135
1. Velocity is a vector which includes speed and direction.

correct.
but the direction such as north or south or whatever
is not used unless needed.

2. Acceleration is change in velocity.

correct.
but over time. ie ... ms^2

3. Change of speed with constant direction = acceleration.

correct.

4. Change of direction with constant speed = acceleration.

correct.

5. A body orbiting at constant speed is constantly accelerating.

correct.

6. Gravity is not a force that holds an orbiting body as though it were on a string.

correct.
gravity is the force that holds a orbiting body.
but not like it was on a string.

to achieve an orbit around a body , gravity is the force
that must be counteracted by the force of the orbiting
objects mass x its velocity.

the velocity for instance is like the tension on the string.
if the orbiting object increases velocity the tension on the string increases , if the string were elastic then the orbiting object would gain altitude.

if the orbiting object decreases velocity the tension on the string decreases , if the string were elastic the orbiting object would loose altitude.

f=mv

for instance a object that normally weighs 1 kg on the earths surface must achieve a orbital velocity that would counteract the pull of gravity that it feels ( 1 kg on earths surface )
but less as the orbit increases , in order to counteract the pull of gravity and maintain a given orbital distance around the body being orbited the orbiting object must achieve a certain velocity in order to achieve an angular acceleration that is the product of the orbiting objects velocity and
the orbiting objects mass and the gravitational pull between the orbited body and the orbiting object that balances the
forces acting on the orbiting object thereby causing the orbiting object to maintain a certain orbital height as it
constantly accelerates towards the earth.

a orbit is simply a balancing act.

7. Gravity alters the geometry of spacetime such that it becomes curved.

no belief = no comment.

8. The curve thus formed is a geodesic, and is defined as the most direct path from A to B in curved spacetime.

no belief = no comment.

9. Thus, a geodesic is equivalent to a straight line in flat (non-curved) spacetime.

no belief = no comment.

10. A body travelling at constant speed in a straight line is not accelerating.

correct.

11. It should be reasonable to argue that a body following a geodesic at constant speed is not accelerating.

incorrect.
you cannot follow a geodesic without accelerating towards
or away from the sections of a geodesic shape as you travel above the sections that make up the geodesic shape.

12. It should, therefore, be reasonable to conclude that an orbiting body is not accelerating.

incorrect.

there can not be an orbiting body that does not accelerate
around a orbited body.

an object will travel in a straight line if it is
not accelerated in another direction away from the straight line.







_________________________
3/4 inch of dust build up on the moon in 4.527 billion years,LOL and QM is fantasy science.

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#55055 - 12/26/15 03:22 PM Re: Do orbiting bodies accelerate? [Re: Orac]
paul Offline
Megastar

Registered: 03/21/06
Posts: 4135
Quote:
If you can't measure an acceleration

Don't mix frameworks as you can't, the definitions of spacetime differ and you will just get confused. Your statement (12) is correct in GR because you can't measure any acceleration with an accelerometer not because of any classical physics calculations. It is true by definition in GR and requires NO calculations.


If you can't measure an acceleration then the measuring tools
that you use are worthless and you should toss them in the
worthless tool bin where they have always belonged. smile

orac , if you were standing on the equator of the earth
right now and holding an accelerometer in your hand and maintaining an exact distance from the ground and standing in the exact same spot.

1) would the accelerometer register any acceleration?

2) would your whole body be accelerating in a direction?

3) if gravity were suddenly removed from the earth would
you remain on the surface of the earth or would you begin
to fly off into space at 1,667,923 METERS PER HOUR ?

or do you wear super glue shoes?

also: since your religion dictates that gravity is due
only to the energy and velocity of a object , just how
do the prophets of your religion explain why your feet
stay on the ground all day long or is it that they intend
to reclaim that the surface of the earth is actually flat
after all and the reason our feet stay on the ground
is because our feet understand how QM , and all the Einstein bullshart works.




_________________________
3/4 inch of dust build up on the moon in 4.527 billion years,LOL and QM is fantasy science.

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#55078 - 01/04/16 02:57 AM Re: Do orbiting bodies accelerate? [Re: paul]
Orac Offline
Megastar

Registered: 05/20/11
Posts: 2819
Loc: Currently Illinois, USA
Paul, you aren't interested in the answer as made clear by your spray at end, so why expect me to waste time answering?

Perhaps measure it all yourself, or ask your GOD what the answer is .. why do I care what a religious fruitloop believes smile

Don't claim you don't have access to an accelerometer there will be a pile of school classroom excercises on the net to build one with a bottle. If you have an I-phone there is a free app that will show you the phones acceleration. So there are two simple options, and who knows maybe even the religious can learn things by experimentation. Feel free to show us what you measure, which I will be happy to discuss.

Now for the record there is widerange accelerometers on board the International Space Station which is orbitting at 27,600 km/h. The reference page is (http://www.nasa.gov/centers/glenn/about/fs11grc_prt.htm) which includes links to the outputs which update every 20 minutes. The instruments are so sensitive they can detect small accelerations from the operation of hardware, crew activities, dockings and maneuvering.

Want to guess what the nominal value the ISS accelerometers measure?


Edited by Orac (01/04/16 03:52 AM)
_________________________
I believe in "Evil, Bad, Ungodly fantasy science and maths", so I am undoubtedly wrong to you.

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#55082 - 01/04/16 05:53 PM Re: Do orbiting bodies accelerate? [Re: Orac]
Bill Offline
Megastar

Registered: 12/31/10
Posts: 1858
Loc: Oklahoma, USA
Originally Posted By: Orac
Want to guess what the nominal value the ISS accelerometers measure?

The ISS accelerometers meassure zero. Because all the components are subjected to exactly the same forces. An accelerometer can only measure the difference in the forces applied to different parts of the accelerometer. Typically an accelerometer uses 2 major components, a 'fixed' body and a sprung mass. The acceleration is calculated as the difference in location between the body and the sprung mass. If both of them are subjected to the same force then they don't sense any acceleration.

Bill Gill
_________________________
C is not the speed of light in a vacuum.
C is the universal speed limit.

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#55083 - 01/04/16 10:34 PM Re: Do orbiting bodies accelerate? [Re: Orac]
paul Offline
Megastar

Registered: 03/21/06
Posts: 4135
Orac
From Bill s's post.

Quote:
12. It should, therefore, be reasonable to conclude
that an orbiting body is not accelerating.


your reply to bill s's #12

Quote:
If you can't measure an acceleration you aren't smile

Don't mix frameworks as you can't, the definitions of spacetime
differ and you will just get confused.
Your statement (12) is correct in GR because you can't measure
any acceleration with an accelerometer not because of any
classical physics calculations. It is true by definition
in GR and requires NO calculations.



my reply to bill s's #12

Quote:
12. It should, therefore, be reasonable to conclude that an orbiting body is not accelerating.

incorrect.

there can not be an orbiting body that does not accelerate
around a orbited body.

an object will travel in a straight line if it is
not accelerated in another direction away from the straight line.




you seem to think or is it that you believe
that you are not undergoing constant acceleration
because you cannot measure that acceleration.

you really do need to go back and first learn the
basics of physics before you try to dump on classical physics.

if your belief or the GR belief believes that ...

Quote:
It should, therefore, be reasonable to conclude
that an orbiting body is not accelerating


then that clearly tells me that your belief and the
belief system of GR is false and invalid.

Quote:
If you can't measure an acceleration you aren't


LOL laugh

Quote:
why do I care what a religious fruitloop believes


your argument always seems to include some type of
anti religious remarks when your knowledge fails to
rescue you from the really stupid situations you get
yourself into because of your lack of knowledge.

try learning the basics of physics orac , it cant hurt
and may actually help you in your quest for the fantastic
that you have submerged yourself into.


_________________________
3/4 inch of dust build up on the moon in 4.527 billion years,LOL and QM is fantasy science.

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#55084 - 01/04/16 11:20 PM Re: Do orbiting bodies accelerate? [Re: Bill]
Orac Offline
Megastar

Registered: 05/20/11
Posts: 2819
Loc: Currently Illinois, USA
Originally Posted By: Bill
The ISS accelerometers meassure zero. Because all the components are subjected to exactly the same forces. An accelerometer can only measure the difference in the forces applied to different parts of the accelerometer. Typically an accelerometer uses 2 major components, a 'fixed' body and a sprung mass. The acceleration is calculated as the difference in location between the body and the sprung mass. If both of them are subjected to the same force then they don't sense any acceleration.

Now lets see if you can extend that with a bit more intelligence than Paul. Assuming you and the accelerometer are in the same reference frame, can you be accelerating if the accelerometer measures zero?

Hint: You probably don't need it but you correctly identified all the forces must sum to zero for the accelerometer to read zero.

What is the classical physics name of this law?

Extension Hint: If we wan't to go deeper with classical physics you are probably going to have to break into linear and non-linear acceleration types and introduce the concept of fictional forces.


Edited by Orac (01/04/16 11:38 PM)
_________________________
I believe in "Evil, Bad, Ungodly fantasy science and maths", so I am undoubtedly wrong to you.

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#55085 - 01/04/16 11:44 PM Re: Do orbiting bodies accelerate? [Re: Orac]
Bill Offline
Megastar

Registered: 12/31/10
Posts: 1858
Loc: Oklahoma, USA
Originally Posted By: Orac
Now lets see if you can extend that with a bit more intelligence than Paul. Assuming you and the accelerometer are in the same reference frame, can you be accelerating if the accelerometer measures zero?

Yes, if you are in freefall. You and all the components of the accelerometer will be subject to the same force. If there are no counteracting forces, such as standing on the floor of an elevator, the accelerometer will not be able to detect the acceleration. Basically the accelerometer can only measure the difference between the net force on the body of the accelerometer and the net force on the sprung mass.

Bill Gill
_________________________
C is not the speed of light in a vacuum.
C is the universal speed limit.

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#55086 - 01/04/16 11:55 PM Re: Do orbiting bodies accelerate? [Re: Orac]
paul Offline
Megastar

Registered: 03/21/06
Posts: 4135
Quote:
Now lets see if you can extend that with a bit more intelligence than Paul. Assuming you and the accelerometer are in the same reference frame, can you be accelerating if the accelerometer measures zero?


oh please let me attempt an intelligent answer to this mr wizard.

Quote:
Assuming you and the accelerometer are in the same reference frame


if myself and the accelerometer are in the same reference
frame then it would certainly appear to me that we were
both accelerating around the planet as we both would observe things such as the stars rising , crossing the sky , and disappearing over the horizon
... this would repeat every day leading me to gain the
knowledge that the earth that I and the accelerometer are
on is not flat and is rotating.

thus from this gathered data I could easily conclude that
I and my accelerometer would obviously be traveling in a circle and that even though we could not measure any acceleration that would in no way mean that we are not accelerating because basic physics math tells me that I and my accelerometer are accelerating , and that there is no need
to include any linear acceleration or any fictional forces to determine that I am accelerating around the planet.

I could simply draw a circle on the ground with a stick.

knowing that I must accelerate the point of the stick
towards the center of the planed circle clearly tells me
that I too must be accelerating ... else the line I draw
would not form a circle.

you always come up with some really stupid questions
when you are found to be wrong in an attempt to cover
up the errors in your knowledge base , if you would have gained
some measure of knowledge before you flooded your
brain with fantasy you might have been capable of not
making so many mistakes that you have to cover up.

_________________________
3/4 inch of dust build up on the moon in 4.527 billion years,LOL and QM is fantasy science.

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#55087 - 01/05/16 12:30 AM Re: Do orbiting bodies accelerate? [Re: Bill]
Orac Offline
Megastar

Registered: 05/20/11
Posts: 2819
Loc: Currently Illinois, USA
Originally Posted By: Bill
Yes, if you are in freefall.

Define freefall please it will help you see the problem you just introduced smile

Perhaps you may care to review Newtons second law (https://en.wikipedia.org/wiki/Newton's_laws_of_motion).

Quote:
Consistent with the first law, the time derivative of the momentum is non-zero when the momentum changes direction, even if there is no change in its magnitude; such is the case with uniform circular motion. The relationship also implies the conservation of momentum: when the net force on the body is zero, the momentum of the body is constant. Any net force is equal to the rate of change of the momentum.

So any suggestions how we should reconcile these two concepts?

So far you have freefall = something that has acceleration yet measures zero net force and yet Newtons second law states "net force on the body is zero, the momentum of the body is constant"

Those two statements are very inconsistent ????????????

You are on the right track but need some better hand waving smile

Hmm lets think about a hint for you: (https://en.wikipedia.org/wiki/Gravimeter) or perhaps google bucket argument

Originally Posted By: Paul
basic physics math tells me that I and my accelerometer are accelerating , and that there is no need
to include any linear acceleration or any fictional forces to determine that I am accelerating around the planet.

Show me the maths ... I sense poor Newtons motion laws going down laugh

Without making fun of you that is why centripetal acceleration is described as a pseudo force or a fictitious force ... google it.

If it helps (https://en.wikipedia.org/wiki/Fictitious_force) and it's one way out of Bill G's predicament.

It's probably different in Paul physics but that is how it is in classical physics because we need to do a patch job to allow all the laws to coexist. If you make the centripetal acceleration real you will violate Newtons laws of motion and all hell breaks loose with your physics.

It will be most interesting to see how both of you tackle resolving this.


Edited by Orac (01/05/16 03:08 AM)
_________________________
I believe in "Evil, Bad, Ungodly fantasy science and maths", so I am undoubtedly wrong to you.

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#55088 - 01/05/16 03:57 AM Re: Do orbiting bodies accelerate? [Re: Orac]
Bill Offline
Megastar

Registered: 12/31/10
Posts: 1858
Loc: Oklahoma, USA
Originally Posted By: Orac
It will be most interesting to see how both of you tackle resolving this.

I personally feel no great need to feed your ego by trying to reconcile your clay pigeons.

Bill Gill
_________________________
C is not the speed of light in a vacuum.
C is the universal speed limit.

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#55089 - 01/05/16 04:22 AM Re: Do orbiting bodies accelerate? [Re: Bill]
Orac Offline
Megastar

Registered: 05/20/11
Posts: 2819
Loc: Currently Illinois, USA
I really don't care , just thought you might be interested to resolve it.

The interesting comment for you in gravimeters you may want to consider

Quote:
Absolute gravimeters, which nowadays are made compact so they too can be used in the field, work by directly measuring the acceleration of a mass during free fall in a vacuum, when the accelerometer is rigidly attached to the ground

Is your definition of free falling the same smile

You may want to consider will an absolute gravimeter measure anything under your definition of free falling.

At the end of the day this is classic physics at it's core and since you like this framework it's interesting to see where it breaks down and what underpins it.

The question posed is ancient going back to Newton and his bucket and when we talk of motion, and by extension change in motion over time (AKA acceleration), we need to be very careful "with Respect to what are we measuring". You blatantly pulled a universal reference frame and I just inverted it on you, so your classic laws come into conflict. Your answer, like all answers to this problem, is sometimes correct in certain reference frames but it isn't a universal solution by any means.

There are as many answers to this problem as there are reference frames and you can't give a definitive answer in classical physics and that was the memo.

An interesting video from the ISS on this is the re-boost operation. Which is free falling during the video the ISS or the astronauts as the ISS is boosting to go back closer to zero gravity condition ... it's lucky you know what is going on !!



One bailed, so lets see what Paul goes with.


Edited by Orac (01/05/16 07:43 AM)
_________________________
I believe in "Evil, Bad, Ungodly fantasy science and maths", so I am undoubtedly wrong to you.

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#55090 - 01/05/16 02:19 PM Re: Do orbiting bodies accelerate? [Re: Orac]
Bill Offline
Megastar

Registered: 12/31/10
Posts: 1858
Loc: Oklahoma, USA
Originally Posted By: Orac
Absolute gravimeters, which nowadays are made compact so they too can be used in the field, work by directly measuring the acceleration of a mass during free fall in a vacuum, when the accelerometer is rigidly attached to the ground

A gravimeter is not an accelerometer. A gravimeter is used to measure the force of gravity at a place on the surface of the Earth. They may include accelerometers but they aren't accelerometers. Don't try changing the rules of the game in the middle.

Bill Gill
_________________________
C is not the speed of light in a vacuum.
C is the universal speed limit.

Top
#55091 - 01/05/16 07:36 PM Re: Do orbiting bodies accelerate? [Re: Orac]
paul Offline
Megastar

Registered: 03/21/06
Posts: 4135
Quote:
Show me the maths ... I sense poor Newtons motion laws going down


I will tell you the elements and methods you will need to know
in order to calculate it yourself but I will include an example.


1) you need to know the acceleration that is the result
of the attraction of gravity between yourself and the earth.

(9.8 ms^2)

the above 9.8 ms^2 acceleration is the force that pulls you towards the earth (980 N) if your mass is 100 kg.

2) you need to know your mass.(kg)

3) you need to know the distance from yourself to the center
of the earths gravity field.

4) I will find the needed orbital velocity.


in order to maintain a orbit around the earth all you need
to do is to match or balance the force of gravity
( the inward pull towards the center of the earths gravity field)
and the force caused by angular acceleration
( the outward force that pulls you away from the center of the earths gravity field due to the angular acceleration of
your body as it travels in a circular path around
the earth)

in essence you need to achieve a certain velocity to do this.

using the centripetal force formula you can calculate
the amount of orbital velocity that you must achieve in order to achieve a orbit with a constant altitude away from the center of the earths gravity field.

F(centripetal) = mv^2/r

if you know your mass equals = 100kg

and you know the force is 980 N ( 9.8 ms^2)

then you need to counteract the acceleration of
a force of 980 N against a mass of 100 kg

980 N will accelerate a 100 kg mass a distance of
9.8 meters in 1 second.

lets say your radius from the earths center of gravity is
in maximum low earth orbit (2000 km above surface )
adding in the 6,371 km radius of the earth you have a total
radius from the center of the earths gravity field of 8,371 km

radius = 8,371,000 meters

using the knowns to find the nedded velocity to establish a orbit.

100 kg
8,371,000 meters radius
9.8 ms^2 acceleration

find velocity

the formula results in a F (centripetal) that is the result of a division
of (mass x velocity squared / radius)
we know that the acceleration is 9.8 ms^2
so we multiply the radius by 9.8

were counteracting the acceleration caused by the force of gravity acting against a 100 kg mass with a force caused
by the resistance of an object to undergo angular acceleration
or simply put the resistance of an object to change its
direction so the result F in the centrifugal force formula
will be given as an acceleration not a force.

8,371,000 meters (radius) x 9.8 = 82,035,800

this gives us the result of mass x velocity squared
so we divide by the mass to get the velocity squared.

82,035,800 / 100 (mass) = 820,358 (velocity squared)

we then find the square root of velocity squared to get the velocity.

sqr of 820,358 = 905.73 ms (velocity)


so your orbital velocity needs to be apx 905.73 ms
in order to stay in orbit around the earth at a orbital
altitude of 2000 km.


smile

_________________________
3/4 inch of dust build up on the moon in 4.527 billion years,LOL and QM is fantasy science.

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#55092 - 01/05/16 11:12 PM Re: Do orbiting bodies accelerate? [Re: Bill]
Orac Offline
Megastar

Registered: 05/20/11
Posts: 2819
Loc: Currently Illinois, USA
Originally Posted By: Bill
A gravimeter is not an accelerometer. A gravimeter is used to measure the force of gravity at a place on the surface of the Earth. They may include accelerometers but they aren't accelerometers. Don't try changing the rules of the game in the middle.

ROFL .... ready

Look at the relative version of gravimeter its described thus
Quote:
Most common relative gravimeters are spring-based. They are used in gravity surveys over large areas for establishing the figure of the geoid over those areas. A spring-based relative gravimeter is basically a weight on a spring, and by measuring the amount by which the weight stretches the spring, local gravity can be measured.


Now lets see how Bill G described his accelerometer
Originally Posted By: Bill G
Typically an accelerometer uses 2 major components, a 'fixed' body and a sprung mass. The acceleration is calculated as the difference in location between the body and the sprung mass.


A GRAVIMETER IS A SINGLE AXIS ACCELEROMETER smile

Originally Posted By: wikipedia
A gravimeter is a type of accelerometer, specialized for measuring the constant downward acceleration of gravity, which varies by about 0.5% over the surface of the Earth. Though the essential principle of design is the same as in other accelerometers, gravimeters are typically designed to be much more sensitive in order to measure very tiny fractional changes within the Earth's gravity of 1 g, caused by nearby geologic structures or the shape of the Earth and by temporal tidal variations.
_________________________
I believe in "Evil, Bad, Ungodly fantasy science and maths", so I am undoubtedly wrong to you.

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#55093 - 01/05/16 11:37 PM Re: Do orbiting bodies accelerate? [Re: Orac]
Bill Offline
Megastar

Registered: 12/31/10
Posts: 1858
Loc: Oklahoma, USA
Originally Posted By: Orac

Quote:
Most common relative gravimeters are spring-based. They are used in gravity surveys over large areas for establishing the figure of the geoid over those areas. A spring-based relative gravimeter is basically a weight on a spring, and by measuring the amount by which the weight stretches the spring, local gravity can be measured.



Originally Posted By: Bill G
Typically an accelerometer uses 2 major components, a 'fixed' body and a sprung mass. The acceleration is calculated as the difference in location between the body and the sprung mass.



But you especially referenced a gravimeter that measured the acceleration of a mass in free fall. Stop trying to change the rules of the game in the middle.

Bill Gill
_________________________
C is not the speed of light in a vacuum.
C is the universal speed limit.

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#55094 - 01/05/16 11:41 PM Re: Do orbiting bodies accelerate? [Re: Bill]
Orac Offline
Megastar

Registered: 05/20/11
Posts: 2819
Loc: Currently Illinois, USA
Originally Posted By: Bill
But you especially referenced a gravimeter that measured the acceleration of a mass in free fall. Stop trying to change the rules of the game in the middle.

There now we have what you don't get smile

It's the same thing .. try work out why ... there is only one way to formulate acceleration.

Hint: Try to find a reference point. Try the old drop big object and little object from a building. So the free falling mass gravimeter is going to read what when in free fall?

Edit: Didn't think but the video above has the astronauts on the ISS free falling. So they are your mass in free fall what acceleration are they showing?

Final Hint: Are Newtons laws (the laws our device is using to measure) valid in a non inertial frame.


Edited by Orac (01/06/16 03:27 AM)
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I believe in "Evil, Bad, Ungodly fantasy science and maths", so I am undoubtedly wrong to you.

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#55095 - 01/05/16 11:56 PM Re: Do orbiting bodies accelerate? [Re: Orac]
Orac Offline
Megastar

Registered: 05/20/11
Posts: 2819
Loc: Currently Illinois, USA
Paul that is a huge post about absolutely nothing ... I am pretty sure I know how to calculate centripetal force. What you haven't addressed is the centripetal force REAL, against what is it pushing to push the orbiting body out.

Go right to the center pivot of any gravitational centripetal setup and the force goes into a single point which is a singularity. Why a singularity because the pivot has a size of zero. Pressure is force per unit area and the pivot has an area of zero.

So Pressure at pivot space = Some force / zero

In other words the pressure on space for any given centripetal setup you just described is INFINITY. We can usually talk around it on physical bodies but now try a spaceship pulling a circle manouvre in empty space.

If you want to test the force and pressure put a weight on the end of a string and swing it around.

So in the physical situation the pivot or axle has width and structural strength. We don't have either of those in the gravity based version.

The very classical formula's you are using will also prove that what is happening can't happen.

That is why centripetal acceleration is called an fictitious force in classical physics, it arises from nowhere and disappears to nowhere. The force is much easier to describe from an inertia frame of reference but in classical physics there is supposed to be an absolute frame and this shows the problem. If I pull an absolute frame then I get an infinity.

Structurally gravity works the same way the force while measurable disappears to nowhere. It's the problem Bill G is struggling with it all sort of makes sense until you try to follow this to an absolute reference and it all blows up in your face and you will get infinity or zero.

What Bill G has not cottoned onto is when he is standing on earth he is in free fall it is just the fall is opposed. When he drops the gravimeter into free fall it will read zero because it is no longer opposed. You can either argue gravity is a fictional force and the gravimeter is just reading the fictional force or you can go the frame of reference is different route like your centripetal acceleration.


Edited by Orac (01/06/16 12:28 AM)
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I believe in "Evil, Bad, Ungodly fantasy science and maths", so I am undoubtedly wrong to you.

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#55096 - 01/06/16 12:23 AM Re: Do orbiting bodies accelerate? [Re: Orac]
paul Offline
Megastar

Registered: 03/21/06
Posts: 4135
Quote:
What you haven't addressed is the centripetal force REAL, against what it is pushing to push the orbiting body out.


yes the centripetal force is real.
it is pushing against gravity.
force vs force.

I know what the problem is that you are struggling with.
its the forces at play.
you cant see the forces or measure the forces so you think that they are not REALLY there.

heres a good experiment you can try for yourself.

get your iPhone out and open up the accelerometer that
is causing you all this grief.

climb up to the top of a ladder and keeping your eyes on
the accelerometer step briskly off the top of the ladder
observing the indicator on the acceleration guage of the
app.

dont look at anything else , only the accelerometer.

1) did the app register an acceleration?
2) did the ground move towards you when you steped off?
3) what pushed the ground up towards you?
4) what pulled you towards the ground if nothing pushed the ground towards you?
5) was it a real force that caused your feet to land back
on the ground?
6) if you didnt measure any acceleration then you didnt accelerate , correct?
7) should you now try the experiment using a tall building
knowing the results from your initial experiment?

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